Typically, nitride-based films and heterostructures are grown using Metal Organic Chemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy (MBE), or Reactive Molecular-Beam Epitaxy (RMBE).
Chemical vapor deposition reactors permit the treatment of substrates, such as wafers mounted on a wafer carrier inside a reaction chamber. FIG. 1 shows an illustrative chemical vapor deposition reactor according to the prior art. As illustrated, a gas distribution injector or injector head 2 is mounted facing towards the wafer carrier 4. The injector 2 typically includes a plurality of gas inlets that provide some combination of gases to the chamber for chemical vapor deposition. Some gas distribution injectors provide a shroud, or carrier gas, that assists in providing a laminar gas flow during the chemical vapor deposition process, where the carrier gas typically does not participate in chemical vapor deposition. Many gas distribution injectors have showerhead designs, in which gas inlets are spaced in a pattern on the head.
A gas distribution injector 2 typically permits precursor gases to be directed from gas inlets on an injector surface towards certain targeted regions of the reaction chamber, where wafers can be treated for processes such as epitaxial growth of material layers. Ideally, the precursor gases are directed at the wafer carrier in such a way that the precursor gases react on the wafer or as close to the wafers as possible, thus maximizing reaction processes and epitaxial growth on the wafer surface.
In many metal organic chemical vapor deposition (MOCVD) processes, for example, combinations of precursor gases comprised of metal organics and hydrides, such as ammonia or arsine, are introduced into a reaction chamber through the injector. Process-facilitating carrier gases, such as hydrogen, nitrogen, or inert gases, such as argon or helium, also may be introduced into the reactor through the injector. The precursor gases mix in the reaction chamber and react to form a deposit on a wafer held within the chamber. The carrier gases typically aid in maintaining laminar flow at the wafer carrier.
Many existing gas injector systems have problems associated with uniform deposition and subsequent uniform epitaxial growth of semiconductor layers. For example, precursor injection patterns in existing gas distribution injector systems may contain regions with low gas circulation, whereas other areas may contain recirculation patterns. In addition, some reaction may happen at the injector walls resulting in unwanted deposition. Such deposition consumes reactants and decreases the efficiency and reproducibility of the process. Moreover, reaction products deposited on the injector or on the reactor wall can be dislodged and can contaminate the substrates.
In a chemical vapor deposition system, uniform deposition on wafer surfaces is of prime importance. In a typical chemical vapor deposition system, one or more wafers may be heated by placing it on a susceptor, and reaction gases are provided into the reaction chamber via gas injection system to initiate the growth of layers on the wafers.
Considerable effort has been devoted in the art to achieve uniform reaction conditions over the entire extent of the wafer carrier to assure that the deposited layers grow uniformly on all of the wafers. Another desire is to assure that the process gases supplied to all regions of the reactor are used efficiently and are not wasted.
Chemical vapor deposition systems in which the wafer carrier is rotated tend to increase deposition uniformity. To this extent, one approach to improve gas circulation in the reaction chamber uses Vertical Rotating Disc Reactors (RDRs), which are widely used for the large-scale production of gallium nitride (GaN)-based semiconductor devices, such as blue and green light-emitting diodes (LEDs), ultraviolet LEDs, solid-state lasers, and heterojunction bipolar transistors. In RDRs, rotation of the wafer carrier results in an effective averaging of the deposition rate distribution, which is a key mechanism providing growth of epitaxial layers with highly uniform properties. FIGS. 2A and 2B show the schematics and gas flow field, respectively, of a reactor chamber including a rotating disk holder according to the prior art.
In addition to the RDRs, approaches have also sought to optimize the shower heads contained in the injector to improve overall uniformity of gas deposition and stability of the flow above the rotating wafer carriers. Furthermore, for systems with wafer carriers holding multiple wafers, the systems have been implemented to rotate the entire wafer carrier set at a first rate, while spinning the individual wafer carriers around themselves at a second rate, thus creating planetary motion of the wafers. FIG. 3 shows a schematic of a system of rotating disk holders undergoing planetary motion according to the prior art. Typically, the wafer planetary motion system is achieved through a set of gears. Mechanical stress in the gear system, together with thermal stress due to high operational temperatures of the reaction chamber, adds to the mechanical stress in the mechanism and results in lower reliability and higher pollution of the reaction chamber through particle contamination coming from the gear system.
Another approach seeks to improve the reaction chamber by, in addition to having rotating wafer plates, injecting the gas through a movable injector device in a radial direction towards or away from the axis of rotation. The approach may further include the step of discharging a second gas with a component of motion in the downstream direction toward the substrates. Numerous factors, including the flux of first gas through the first movable gas inlet and the speed of motion of the first movable gas inlet, are allowed to be controlled so as to control the rate of reaction as, for example, the rate of deposition in a chemical vapor deposition reaction, as well as the composition of the deposited material. In addition, the approach also describes a gas injector head wherein the injector head includes a body having a central axis, the body being arranged for mounting to the reactor with the central axis aligned with the axis of rotation of the wafer carrier in the reactor, and with the head facing in a downstream direction.